1. Field of the Invention
This invention relates to a method for controlling the quantity of intake air supplied to an internal combustion engine, and more particularly to a method of the kind described above by which a fuel-air mixture having a proper mixture ratio can be continuously supplied to the combustion chambers of the engine under whatever operating condition of the engine.
2. Description of the Prior Art
An internal combustion engine equipped with an electronic controlled fuel ignition system (abbreviated hereinafter as an ECI) is commonly provided with an idle speed control device (abbreviated hereinafter as an ISC) as a means for controlling an idling rotation speed of the engine. The ISC acts to electrically control the quantity of a fuel-air mixture supplied to the combustion chambers of the engine independently of the amount of depression of the accelerator pedal, so that the idling rotation speed of the engine can be maintained at an appropriate level. The ISC is broadly classified into two types, that is, a direct actuation type and a bypass type. In the ISC of the direct actuation type, a throttle valve is directly actuated by an actuator. On the other hand, in the ISC of the bypass type, a bypass passage is provided in parallel to an intake pipe, and the sectional area of the bypass passage is increased or decreased by an associated valve member.
FIG. 3 shows one form of the ISC of the bypass type. Referring to FIG. 3, an ISC valve 1 acting as a flow control valve is directly mounted on one side of an intake pipe 2. In a valve body 101 of this ISC valve 1, an intake air bypass passage 102 communicating with both an upstream side and a downstream side of a throttle valve 3 in the intake pipe 2 is formed, and a valve seat 103 is press-fitted in a part of the valve body 101 defining the downstream end of the intake air bypass passage 102.
A stepping motor 106 consisting of a stator 104 and a rotor 105 is mounted on the top of the valve body 101, and a valve shaft 107 is coupled at its upper end to the stepping motor 106 through a feed screw mechanism in the form of a screw threaded spline connection. A valve member 108 is formed on the lower end of the valve shaft 107. This valve member 108 is located opposite to the valve seat 103 and makes vertical movement relative to the valve seat 103 thereby increasing and decreasing the sectional area of the intake air bypass passage 102. A coil spring 109 is mounted on the valve shaft 107 so as to eliminate backlash of the feed screw mechanism.
The operation of the ISC valve 1 is controlled by an electronic control unit (abbreviated hereinafter as an ECU) (not shown) in a manner which will be briefly described below.
As compared to idling under a no-loaded condition of the engine, it is necessary to supply the fuel-air mixture of a larger quantity to the combustion chambers of the engine to drive an auxiliary equipment such as a coolant compressor of an air conditioner, an alternator or an oil pump of a power steering mechanism, in order to maintain the same rotation speed of the engine. Also, in the case of fast idling for the purpose of warming up the engine, the fuel-air mixture in a quantity sufficient for maintaining the high rotation speed is required.
The ECU determines the idling rotation speed of the engine on the basis of various operation parameters. For example, on the basis of an output signal of a crank angle sensor, the ECU applies a control signal to the ISC valve 1 for controlling feedback of the engine rotation speed (abbreviated hereinafter as N-feedback) so as to maintain the idling rotation speed. However, the N-feedback described above cannot sufficiently deal with a sharp increase or decrease in the load due to starting or stopping the operation of an auxiliary equipment of the kind described above, and delayed control results in undesirable stalling of the engine or blow-up. Therefore, in such a case, valve position feedback control (abbreviated hereinafter as P-feedback) is carried out in which the opening of the ISC valve 1 is determined on the basis of the factors including the amount of the load, and the ISC valve 1 is controlled to be set at the determined valve opening. This P-feedback mode is the so-called predictive control mode and shifts to the N-feedback mode as soon as the engine starts to operate in its steady operation state.
In each of the N-feedback mode and the P-feedback mode, the ECU supplies a pulse current to the stepping motor 106 after determination of the engine rotation speed to be controlled. In response to the supplied pulse current, the rotor 105 of the stepping motor 106 rotates through a predetermined number of steps to cause vertical movement of the valve shaft 107 thereby changing the position of the valve member 108 relative to the valve seat 103. As a result, a change occurs the quantity of intake air flowing from the upstream side toward the downstream side of the throttle valve 3 while flowing through the bypass passage 102 in the ISC valve 1.
In the air intake system described above, air instead of a fuel-air mixture flows through the bypass passage 102 in the ISC valve 1. That is, the above description refers to a multipoint injection system in which a plurality of injectors are disposed on the downstream side of the throttle valve 3. However, in the case of a single-point injection system where a single injector is disposed on the upstream side of the throttle valve 3, a fuel-air mixture instead of air flows through the bypass passage 102 in the ISC valve 1.
The valve opening of the ISC valve 1 is determined depending on the position of the valve shaft 107 relative to the valve seat 103. In the case of the P-feedback mode, the position of the valve shaft 107 is controlled on the basis of the number of driving steps (abbreviated hereinafter as steps) of the stepping motor 106 from the full closed position of the ISC valve 1. Suppose, for example, that the engine is idling under a no-loaded condition after being warmed up (referred to hereinafter as hot idling), and the valve opening under the hot idling condition (referred to hereinafter as hot-idling basic opening) corresponds to 9 steps. When, for example, the air conditioner or the alternator is driven during the hot idling condition of the engine, the corresponding valve opening (referred to hereinafter as load-dependent opening) must be increased by 5 steps from the number of steps in the hot-idling basic opening. Also, when the power steering mechanism is additionally actuated, the load-dependent opening must be increased by 10 steps. Further, the valve opening under a fast idling condition (referred to hereinafter as fast-idling basic opening) is as large as a number of, for example, 50 steps. Thus, depending on the load imposed on the engine rotating under the hot idling condition, the valve opening is compensated or increased in the manner described above. That is, the valve opening is determined to meet the load imposed on the engine.
FIG. 7 is a map representing the relation between the number of steps required for opening the ISC valve 1 from the full closed position and the corresponding quantity of air flowing through the bypass passage 102 in the ISC valve 1. It will be seen from this map that there is a progressive relation between then. The reason why the ISC valve 1 has such a non-linear opening characteristic will now be described.
When the engine is operating in the hot idling condition, it is necessary to delicately control the rotation speed of the engine so as to stabilize the rotation and to maintain satisfactorily low fuel consumption. For this purpose, it is necessary to minimize the change in the quantity of air supplied per step, that is, to minimize the gain. On the other hand, when, for example, the power steering mechanism is actuated, undesirable stalling of the engine due to an overload tends to occur unless the quantity of air supplied to the engine is sharply increased. Also, when the engine is to be started from its cold condition, the opening of the ISC valve 1 must be increased from the hot-idling opening to the fast-idling opening within a very short period of time so as to avoid trouble such as mal-starting or impossibility of starting. Therefore, the gain is progressively increased from that under the hot-idling condition toward that under the full open position of the ISC valve 1 to satisfy all of those requirements.
The valve opening of the ISC valve 1 in the P-feedback mode is controlled by controlling the number of steps of rotation of the rotor of the stepping motor 106. However, the prior art manner of valve opening control has had a problem which will be described below.
The aforementioned valve opening compensation under the loaded condition (referred to hereinafter as load-dependent compensation) is determined on the basis of the hot-idling basic opening.
It will be seen in FIG. 7 that the basic opening and the quantity of air supplied under the hot idling and no-loaded condition are 9 steps and 0.4 g/sec respectively. Then, when the handle is manipulated with the aid of the power steering mechanism while driving both the air conditioner and the alternator from the above condition, the opening of the ISC valve 1 is increased by 20 steps to be now set at the value corresponding to 29 steps, and the quantity of air is now set at 1 g/sec. That is, the quantity of air for the load-dependent compensation is incremented by 0.6 g/sec.
However, in the fast idling mode of the engine, the basic opening and the quantity of air supplied under the no-loaded condition are set at 50 steps and 2.3 g/sec respectively as shown in FIG. 7. Then, when the handle is manipulated while driving both the air conditioner and the alternator as in the above case, the opening of the ISC valve 1 is increased by 20 steps to be now set at 70 steps, and the quantity of air is now set at 5.0 g/sec.
Thus, in spite of the fact that the required increment of the quantity of air to attain the load-dependent compensation is 0.6 g/sec, the quantity of air is now incremented by 2.7 g/sec, and this means that the excess quantity of air as large as 2.1 g/sec is now supplied. This will result in such an undersirable situation that difficulty is encounted for maintaining the rotation speed of the engine at the required level.